Serum osteocalcin has been generally regarded as an index of osteoblastic activity. There are many studies which use circulating osteocalcin concentrations in the assessment of bone turnover and for the monitoring of response to therapy. However, there are several unanswered questions which must be resolved before there can be complete confidence in the information gained from its measurement. 1. What is the nature of the circulating form(s) of osteocalcin? Studies suggest that there are several immunoreactive forms of osteocalcin in the circulation. This proposal will identify and isolate the circulating form(s) of osteocalcin by using a series of monoclonal antibodies produced to various regions of human osteocalcin. this will be done in: a) serum from normal adults and children, both males and females, across all age ranges; b) patients with metabolic bone disease, i.e., 1o and 2o hyperparathyroidism, hyper- and hypothyroidism, Paget's disease, osteomalacia and rickets, and osteoporosis; c) during dynamic testing, and d) from isolated osteoblasts. 2. What are the factors which regulate the concentration of osteocalcin in the blood? Studies indicate that serum osteocalcin concentrations can be regulated by acute changes in extracellular calcium. these studies will examine the acute effects of altered calcium homeostasis on circulating osteocalcin. This will be done by investigating the effects of changing mineral concentrations on circulating osteocalcin in intact, thyroparathyroidectomized, and hindlimb perfused rats? 3. What is the source of circulating osteocalcin? Animal studies indicate that serum osteocalcin is derived from new protein synthesis rather than from release of existing osteocalcin from matrix. However, this is inconsistent with acute fluctuations in serum osteocalcin with changes in CaI. The proposed studies will identify the source of circulating osteocalcin in the basal state and under altered conditions. This will be done by investigating the effects of changing CaI concentrations on circulating osteocalcin in acutely warfarinized rats. 4. What is the function of osteocalcin in bone? These studies will examine the potential role of osteocalcin in calcium homeostasis. This will be accomplished by dynamic hormonal and mineral challenges in chronically warfarinized rats. These studies will clarify the interpretation of osteocalcin measurements and to define the role of osteocalcin in calcium homeostasis.